Expandable thermoplastic beads, e.g. polystyrene beads, are commercially produced by a suspension polymerization where liquid styrene monomer is dispersed in an aqueous medium containing one or more suspension agents, a hydrocarbon blowing agent and a polymerization catalyst. The polymerization is often performed at temperatures above 70.degree. C and sometimes above 100.degree. C in order to reduce the amount of residual monomers in the beads.
The obtained beads comprise polymer shells containing the liquid, volatile blowing agent. The beads expand by being heated to a temperature above the boiling point of the blowing agent and above the softening point of the polymer. The beads can be used for the production of cellular material, which finds many uses in the insulation industry or packaging industry. As examples of other special uses there can be mentioned the production of paper or paperboard having a low density as described in the U.S. Pat. No. 3,941,634.
In the initial stage of the polymerization, e.g. in the case of expandable polystyrene beads, the styrene and the blowing agent, pentane or another hydrocarbon, form a droplet with only one phase. In a later stage of the polymerization the blowing agent is not soluble in the polymer phase and builds a separate phase in the form of small inclusions in the polymer droplets. In the initial stage the pressure may be 100 - 200 kPa. The pressure increases during the polymerization to 300 - 500 kPa depending on the blowing agent and the temperature. The high concentration of blowing agent inside the beads and the high temperature produce a considerable pressure inside the beads, and they expand in the unfilled or gas-filled part of the reactor. The unfilled or gas-filled part of the reactor is referred to as the "free volume" in the following description. The "free volume" is further increased during polymerization because of an increase in density of about 20% in the conversion from monomer to polymer.
The pre-expanded beads are a problem and increase the cost of production. The pre-expanded beads have to be separated by filtration or by other means from unexpanded beads, and the pre-expanded beads increase the amount of waste.
Attempts have been made to avoid the pre-expansion of the beads by applying a high pressure e.g. 500 - 1500 kPa of an inert gas (e.g. nitrogen) during the polymerization. However, this does not eliminate the pre-expanded beads. The gas does not actually apply a higher pressure on the outside of the beads. An equilibrium between the gas inside the beads and the gas in the "free volume" in the reactor is established. The beads continue to expand because of the "free volume". and the high pressure of hydrocarbon inside the beads.